Supplemental Financial Information Quarterly Results Supplemental Financial Information - Schedule of Unaudited Quarterly Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 76,300	$ 44,600	$ 86,700	$ 79,600	$ 92,300	$ 62,700	$ 47,300	$ 82,700	$ 287,152	$ 285,079	$ 293,993
Total expenses	55,200	41,600	61,300	57,900	64,200	48,800	43,800	57,900	215,946	214,604	225,323
Income before taxes	21,100	3,000	25,400	21,700	28,100	13,900	3,500	24,800	71,206	70,475	68,670
Provision for taxes	5,300	1,200	9,900	8,100	13,000	5,300	1,300	8,700	24,524	28,383	24,086
Net income allocated to common stockholders	$ 15,800	$ 1,800	$ 15,500	$ 13,600	$ 15,100	$ 8,600	$ 2,200	$ 16,100	$ 46,682	$ 42,092	$ 44,578
Basic (usd per share)	$ 0.53	$ 0.06	$ 0.52	$ 0.45	$ 0.50	$ 0.28	$ 0.07	$ 0.53	$ 1.55	$ 1.38	$ 1.44
Diluted earnings per share	$ 0.53	$ 0.06	$ 0.52	$ 0.45	$ 0.50	$ 0.28	$ 0.07	$ 0.53	$ 1.55	$ 1.38	$ 1.44
Dividends declared per share (usd per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.80	$ 1.80	$ 1.80